The present invention relates to display of color images on mobile phones, computer monitors, and other types of devices, and more particularly to adaption of color images to ambient light.
Displayed images can be hard to see in bright ambient light due to screen glare. FIG. 1 illustrates a human user 110 watching the screen of a mobile phone 120 having a display 130. The images are generated by the display 130 emitting light 140. In addition, the display reflects ambient light 150 generated by ambient light sources 160 such as the sun or office lighting. The reflected light 150R can overpower the display's own light 140, and the user 110 may be unable to clearly see the images, especially dark image areas.
Some self-emissive liquid crystal displays (LCDs) automatically increase their backlight power in response to increased ambient light 150. Specifically, a self-emissive LCD 130 has a backlight source behind the screen. The backlight source emits the light 140 towards user 110. The light passes through liquid crystal cells, whose transmissivity is controlled by electrical signals to form an image. The image can be brightened by increasing the backlight power. In some displays, the backlight power is automatically increased if an ambient light sensor 180 in device 120 senses increased ambient luminance. However, the backlight power cannot be increased indefinitely due to engineering limitations. Therefore, even the maximum backlight power may be insufficient to comfortably see the screen in bright ambient light. Also, power must be conserved in some devices, especially if they are battery-operated. Further, high backlight brightness reduces the dynamic range of the display.
Alternatively, dark images areas can be brightened by making the corresponding pixels more transmissive. See U.S. Pat. No. 9,530,342, issued 27 Dec. 2016 to Bell et al., incorporated herein by reference. The Bell patent proposes modifying the image “dominated by darker tones” by boosting the luminance of darker tones “in proportion to the estimated ambient brightness”. This boosting is achieved by “modifying or selecting one or more gamma curves associated with the display” based on the ambient light. A gamma curve “may take the form of a power function”. As is well known, a gamma curve may define a transformation of a color coordinate (e.g. red, green, or blue intensity value) before that coordinate is converted to an optical signal by the display. See also U.S. pre-grant patent publication no. 2018/0040297 A1 (applicant: Dunn et al.), incorporated herein by reference.
Alternative image adaptation to ambient light is desirable.